1. Field of the Invention
The present invention relates to a sheet stacker/conveyor and an image forming apparatus using this sheet stacking/conveying unit, e.g., an electronic copying machine.
2. Description of the Related Art
General image forming apparatus such as electronic copying machines, printers, etc. incorporating an image forming unit in the inside are demanded to print images not only on the surfaces of sheets but also on the backs in many cases. So, a so-called automatic duplex sheet feeder is provided to stack sheets with images printed on the surface temporarily and then, convey these stacked sheets again to an image forming unit.
This automatic duplex sheet feeder is capable of printing both sides of a sheet by taking a sheet with an image printed on its surface in this unit, turning it over and conveying it again to an image forming unit.
In addition, this automatic duplex sheet feeder is also capable of printing both sides of a plurality of sheets by feeding a plurality of the surface printed sheets in order. Two systems are so far adopted for taking out a plurality of sheets in order.
As a first system, a plurality of sheets stacked in a stacker of an automatic duplex sheet feeder are fed in order from the top sheet. In this system, an image is first printed on the surface of a sheet and this sheet is once stacked in the stacker in this sheet feeder. In this case, the sheet with an image printed on its surface is stacked in order on a sheet with an image printed on the surface previously. Then, after completing the image printing on the surface of a last sheet, the image printing on the back starts from the last sheet with an image printed on its surface.
When printing an image on the back, sheets are fed in order from a highest sheet with an image printed on the last surface and an image is printed on the back. That is, an image is printed on the back in the sequence entirely reverse to the first image printing on the surface.
The printing according to a second system up to the step when a sheet with an image printed on the surface is next put over a sheet with an image printed on the back in the automatic duplex unit is the same. In the image printing on the back, a lowest sheet is taken out and fed for the image printing.
So, according to this type of system, a sheet with an image first printed on the surface is taken out of a stacker in the same sequence as when it was stacked in the stacker.
In the first system, that is in a system to make the image printing on the back by taking a top sheet with an image printed on the surface, productivity of the duplex printing is worse because the image printing on the back cannot be started unless the image printing on the surfaces of all sheet is completed.
In the second system, that is, in a system to make the image printing on the back by taking a lowest sheet with an image printed on the surface, it is possible to take out a sheet from the stacker for the image printing on the back without waiting the completion of the image printing on the surfaces of all sheets and productivity can be increased higher than the first system.
However, in the automatic duplex unit of the second system, it is necessary to perform the operation to stack sheets with an image printed on the surface in the duplex unit and the operation to feed sheets for the image printing on the back by taking them out of the stacker simultaneously and smoothly in order to maintain said high productivity.
So, there is so far available a unit that is able to take out sheets stacked in the automatic duplex unit from the lower surface side by a pick-up roller and immediately feed them for the image printing on the back. However, in such the structure, while sheets are taken out by the pick-up roller, other sheets with an image printed on the surfaces are conveyed to the sheet stacker successively. Therefore, a pressing force required to convey sheets could not be applied to the pick-up roller.
If a pressing force required cannot be applied to the pick-up roller, sheets cannot be surely conveyed and such problems as improper sheet feeding can be generated. In a conventional apparatus, as a countermeasure, such a construction is adopted that a sheet stacker is tilted so that stacked sheets could reach a sheet feed roller by their tare.
In this case, even when the conveying force of the pick-up roller is insufficient, it is possible to surely convey sheets to the sheet feeding roller. On the other hand, because a sheet stacker is tilted, an automatic duplex unit becomes a large size and the entire image forming apparatus becomes a large size accordingly.